alt Hacker News+1 to pavel_lishin's Splendor suggestion. There's also Splendour: Duel [1] which is a more complex version of the game designed for 2 players.
Another quick, low-complexity game that is easy to teach & pretty good fun is Century: Spice Road [2]
Chinatown [3] (re-themed as Waterfall Park [3b] ) is a simple highly interactive game that is basically 100% negotiations between players who are trying to make real estate deals with each other. Can be played in 90 minutes, including rules explanation, plays up to 5. For a more complex asymmetric game that's more focused on engine building, with a healthy dose of negotiation, check out Sidereal Confluence [4].
For more complex games that take a bit longer to play to teach and play, that are largely focused on players doing their own thing ("multiplayer solitaire"), building their engines without much negative player interaction, check out Ark Nova [5] or Terraforming Mars [6]. These might take 3-4 hours or so to finish, provided there's an experienced player to teach everyone the rules.
For another moderately complex strategy game with a little more player interaction, check out Brass: Birmingham [7]. Takes around 4.5 hours to finish a 4 player game, including the rules explanation. If you have a group that enjoys complex strategy games and wants something with spikier negative player interactions, where one player's actions can completely wreck another player's plans, check out Barrage [8].
This probably doesn't help "without spending much money"! One trick is to find or create a regular board gaming group where everyone brings along different games. That way if, everyone buys a new game or two every year there's a lot of variety without everyone needing to buy heaps of games.
[1] https://boardgamegeek.com/boardgame/364073/splendor-duel
[2] https://boardgamegeek.com/boardgame/209685/century-spice-roa...
[3] https://boardgamegeek.com/boardgame/47/chinatown
[3b] https://boardgamegeek.com/boardgame/396618/waterfall-park
[4] https://boardgamegeek.com/boardgame/202426/sidereal-confluen...
[5] https://boardgamegeek.com/boardgame/342942/ark-nova
[6] https://boardgamegeek.com/boardgame/167791/terraforming-mars
[7] https://boardgamegeek.com/boardgame/224517/brass-birmingham
This is the kind of positivity that I love finding find once going down the rabbit hole of board games today.
So make amazing suggestions in this list, including two of my favorites: Terraforming Mars and Brass Birmingham.
Just chiming an opinion that Brass Birmingham is high on the complexity scale for beginner board gamers. Or more specifically, high on a frustration scale because there are so many placement restrictions that there are often only 1-2 legal moves to play and figuring out what they are is quite a challenge for people playing the first time. (From experience that we as well as several others we know had on their respective first times)
That said, I absolutely love the game!